1. Field of the Invention
This invention relates to an implantable medical device for stimulating a heart in vivo of the type including a stimulation pulse generator which generates and delivers stimulating pulses to the heart, an activity sensor which emits an activity signal in response to the living creature's physical activity and a control device which, when the stimulation rate exceeds a preset threshold value, controls the rate at which the stimulation pulse generator delivers stimulating pulses, whereupon every activity signal value is transformed into a stimulation rate signal value by a preset response amplification in the control device.
2. Description of the Prior Art
A prior art example of a related device is disclosed in U.S. Pat. No. 4,428,378 which describes a sensor-controlled pacemaker. An activity sensor transforms sensed mechanical activity into an electric signal which is filtered and amplified by a response amplification so a value is obtained for the stimulation rate at which the heart is to be stimulated. The stimulating pulses are delivered by a stimulation pulse generator which employs the amplified activity signal as an input signal. The response amplification governs the extent to which the stimulation rate is to increase or decrease in response to the pacemaker patient's increased or decreased physical activity. Changes in the stimulation rate are made in the interval between a minimum and a maximum rate, even if the response-amplified signal corresponds to a slower or faster stimulation rate. In this instance, response amplification is linear, and a physician programs the pacemaker with the curve slope he or she feels is suitable for the patient. Prior to this programming, the patient normally performs a brief exercise program to provide the physician with a starting point for the curve slope choice.
Numerous factors, however, can cause the patient's rate during the exercise program to be an inaccurate predictor of the rate the patient will require in daily life. For example, the patient might be ill, or have just recovered from an illness, at the time the response amplification is to be decided. As a result of anxiety, for example, the patient may also be consciously or unconsciously reluctant to perform the exercise program using her/his normal capacity.
Regardless of the reason, the patient's pacemaker may be programmed with response amplification less than ideal for the patient. Depending on the frequency of follow-up appointments, a considerable period of time could elapse before the pacemaker is re-programmed.